This invention relates to a method of applying coatings to metal substrates, utilizing direct contact with a flat, non-deformable applicator roll; and to substrates coated by such a method. In this invention, the metal substrate is metal sheet, not metal foil.
Roll coating has been used for decades to apply liquid coatings to solid substrates for decoration, protection, and functionality. During that time, line speeds have increased, coating uniformity requirements tightened, environmental concerns heightened, and productivity goals raised. Roll coating has evolved according to the industry and coating requirements. Roll configurations vary tremendously; some of the variants are the number of rolls (two to five or more rolls per coating), the position of metering roll(s), coating feed location, coating against a backup roll or sheet tension, roll compositions, and the roll directions relative to each other and the substrate. Wet film thickness varies from 0.05xe2x88x92 greater than 100 micrometers and line speeds from 1xe2x88x92 greater than 500 m/min. Roll coating is used on paper, metal, plastic, and other substrates.
Liquid coatings are applied to substrates in a variety of methods including spray, dip, roll, knife, electrodeposition, vapor deposition, slot, and curtain coating. For metallic substrates, liquid coating is commonly transported to the applicator roll via direct contact with a bath, by contact with a second roll that has contact with a bath, by direct spraying onto the roll, and the like. The rolls can have a metal, plastic or other type surface depending on the material to be coated. The various rolls in a roll coater are usually given common names associated with their function. In the three roll configuration shown in FIG. 1 of this invention, the applicator roll 20 applies the coating to the web, and the backup, or impression roll 22 provides support for the web. If the back-up roll is deformable (rubber-covered) it is an impression roll while a non-deformable roll is a backup roll. The pickup/metering/fountain roll 18 lifts coating from the pan and meters coating with applicator roll 20. Since roll 18 has two functions, it can be called by more than one name. In addition, different industries commonly use different names for the same roll function, hence roll 18 is also called a fountain roll in the printing industry. Depending on the number of rolls in the configuration, additional terms such as transfer roll and spreading roll are commonly used to describe the roll function in a specific industry. The narrow gap between roll 18 and roll 20 is termed the metering nip and the narrow gap between roll 20 and the web is termed the coating nip. The back-up roll supports the web at the point of coating application. If space is maintained between the applicator roll and the web, the back-up roll is usually rigid, and there is no metal-to-metal contact. If the backup roll is forced into near-contact with the applicator roll, one of the two rolls are covered with deformable material to prevent metal to metal contact. Surfaces that move in the same direction at the point of nearest contact (rolls 18 and 20) are said to be moving in a forward direction, and those in opposite direction (roll 20 and the web) in a reverse direction. For the most uniform surface appearance, the applicator roll moves in a reverse direction for continuous web coating. A description of various roll combinations and descriptions of fluid dynamics can be found in Liquid Film Coating, Ed. S. F. Kistler et al. xe2x80x9cKnife and Roll Coatingxe2x80x9d by Dennis J. Coyle, pp 539-542 (1997).
Ceramic surfaced metering rolls, having surface indentations to contain ink and the like, have long been used in the printing industry, as taught in U.S. Pat. Nos. 4,301,730 and 4,009,658 (Heurich et al and Heurich respectively). In Heurich the hard ceramic coating protects the roll from wear and permits a flatter cell profile. Other possible uses for this roll include xe2x80x9cglue application rolls, rotogravure coating rolls and the likexe2x80x9d but the substrates mentioned are compliant paper products. These inventions were improvements over rolls having a plated metal surface. In offset printing processes, water and ink are alternately fed to a plate surface where the ink selectively adheres to the picture portion. Yokoyama et al., in U.S. Pat. No. 4,991,501, used a ceramic faced dampening water feed roll with a flame sprayed layer of Al2O3 and/or TiO2, and also containing SiO2 as a hydrophilic, inorganic pore-occluding agent within ceramic surface occlusions. These rolls were seen as an improvement in terms of better wettability over flame sprayed Cr2O3 roll surfaces. The dampening water feed roll here is not in direct contact with substrate being coated or the plate cylinder.
In the area of coating metallic substrates for packaging and architectural end uses, metallic substrate are coated in one of two ways: as a continuous web or as approx. one m2 (10 ft2) sheets. A continuous xe2x80x9cwebxe2x80x9d is rewound into a coil for subsequent trimming, slitting, or fabrication. Aluminum and steel are most commonly used as the substrate. Many coatings are applied on metal webs via forward or reverse roll coat with a deformable applicator roll. The applicator roll has a pliable covering about 5-50 mm thick made from polyurethane, EPDM rubber or similar pliable material. The covering Shore A hardness, referred to as durometer, is about 40-80 and the surface roughness Ra is about 20-80 micro inches. (Ra is determined by first finding a mean line parallel to the general surface direction, dividing the surface such that the sum of the areas formed above the line is equal to the sum of the areas formed below the line, and computing the surface roughness summing the absolute values of all the areas above and below the mean line and dividing by the sampling length.) This allows the applicator roll to transfer liquid coating to the metallic web and conform to any web surface irregularities or roll irregularities to ensure complete coverage at a uniform film thickness.
Deformable (rubber or polyurethane) applicator rolls are subject to wear and cause mottling, blisters, skips and eyehole defects due to swelling, burnishing, or exhibition of internal defects. When applicator rolls start to produce coating defects, the whole production line must be shut down and the applicator roll replaced. In addition to the cost of resurfacing the roll and finished product scrap, problems associated with stopping and starting a continuous line come to bear. Each line stoppage results in scrap generation and increases the chances for sheet breaks and dents. Another large cost is lost production time.
Two common applicator roll defects are mottling and edge blisters. Mottling occurs from wet coating thickness variations caused by the applicator roll. The applicator roll is thought to pick up oxides from the metal surface or become burnished from repeated contact with the metallic substrate. When a burnished applicator roll receives coating from the metering roll, the burnished surface does not carry as much coating and a low coating weight area results.
Edge blisters are another coating defect that results from deformable rolls wearing at the edge of the web. As the deformable roll covering deforms around the web edge, the web cuts the roll. After switching to a wider web, the cut in the applicator roll leaves an uneven amount of coating on the substrate, and the roll needs to be replaced or resurfaced before production can continue.
Other application problems with deformable applicator rolls, especially in high gloss waterborne colors, are small (1 mmxc3x9710 mm) areas of low coating weight and flow lines. The small areas are higher gloss than the surrounding coating and are eliminated only with difficulty by adjusting roll speeds and nip pressures. Flow lines, or ribbing instability from a forward metering roll nip, are unstable when using a deformable applicator roll.
A second mechanism for edge defects is called roll setxe2x80x94the tendency of the roll covering to retain its compressed dimensions after a stress has been removed. The deformable applicator roll has different compressive stress where the web contacts the web versus where it contacts the backup/impression roll or air. When switching to a wider web, the roll set leaves a step change in roll diameter and a resultant step change in coating weight at that point. The coating xe2x80x98linexe2x80x99 on the web requires immediate roll removal.
Deformable rolls are easily damaged by an inadvertent bump or cut, can contain bubbles or solid contaminants on or immediately under the surface, or collect oxides or organics from the metal web surface. These conditions are known to cause coating non-uniformities that require removal of the applicator roll. Unfortunately, it is very difficult to detect roll imperfections before installation and expensive in terms of time and materials when a roll fails, especially when a minor defect goes unnoticed for several hours production time.
The coating industry has also tried steel metering rolls with steel pickup/applicators but theses systems require a doctor blade, a deformable backup roll, high coating variability limits, or stringent safety procedures. Doctor blades can trap dried coating or contaminants and transfer the resulting coating defect to the web. A deformable backup roll is subject to the same wear issues related to handling varying widths of metallic webs and swelling from exposure to the coating.
Coating weight is controlled by the gaps between the rolls. If the coating thickness is less than 50 micrometers and coating uniformity is important, the total indicated runout of the rolls and bearings is significant to product quality. As roll length and weight increase, roll deflection further negatively impacts coating uniformity. Safety is always an issue when two non-deformable rolls are turning in close proximity at high speeds. Coating starvation can lead to heat generation and coating ignition, and roll misalignment can lead to metal fatigue and projectiles at failure.
Another problem with the current coating processes is the ability to quickly change from one width substrate to another. Currently, coaters start with the widest width customer web and work down to the narrowest width with the same coating system. This practice is not efficient because excess product inventory costs money via material costs, cancelled orders, and handling damage.
Deformable applicator rolls are currently chosen for a variety of properties, usually a balance between resistance to cuts from sharp metal edges, durometer for the proper xe2x80x98footprintxe2x80x99 on the substrate for smooth coating application, resistance to edge set to allow coating at both wide and narrow web widths, and resistance to burnishing from incidental contact with the metallic web. Unfortunately, deformable applicator rolls are optimized for one property at the expense of another, resulting in application problems via one of the above conditions.
One attempt to solve problems such as those described above is taught in U.S. Pat. No. 4,967,663 (Metcalf), which relates to metering rolls for depositing measured amounts of a liquid coating, such as ink or a protective material, on moving objects in a continuous process, where the objects are metal objects, such as cans. The rolls have a thick top of very porous, sintered ceramic, where incorporating organic filler and then burning the organic out during a ceramic sintering operation form the pores. This process solved problems associated with, among other prior operations, chrome-plated, engraved metal metering rolls. One advantage of the deliberately xe2x80x9cpre-poredxe2x80x9d ceramic rolls is that the formed pores are homogeneously distributed, so that simple regrinding provides a new useable porous surface. However, commercial experience has shown that a ceramic applicator roll in close proximity to the non-deformable can insert (supports can wall during the coating process) is not practical because of potential contact between the ceramic roll and can insert.
Heat dissipation is another problem with deformable applicator roll coverings. In addition to heat from shearing the coating in the coating nip (and metering nip if applicable), the flexing of the cover also generates heat. As one uses higher viscosity coatings, especially those containing materials that swell or chemically attack the roll covering, the heat generation increases and exacerbates the changes to the roll covering. Attempts to compensate for these changes, such as higher nip pressures, accelerate applicator roll failure.
Most of these patents do not describe how the coating liquid is applied/transferred to the roll for application on the substrate. Yokoyama et al., U.S. Pat. No. 4,991,501 described previously, immersed the master ceramic water discharge roll directly into the liquid and in contact with two non-immersed rubber rolls, one a metering roll and the other a water applying roll. In another application method, the water discharge roll is again immersed directly into the liquid and mounted under three additional rolls, one of rubber, contacting the immersed roll and another of rubber contacting the surface to be coated. FIGS. 3 and 4 of Yokoyama et al. illustrate a four-and five-roll configuration with a rubber-covered applicator roll.
In U.S. Pat. No. 5,548,897 (Link) an ink transfer roll having an ink metering, engraved Al2O3 surface, optionally overlaid with a ceramic such as Cr2O3, is in direct contact and immersed in the ink bath. The transfer roll is not in direct contact with the article being coated which is typical of many roll-coating operations. In U.S. Pat. No. 6,341,559 B1 (Riepenhoffet al.) an inking roll transfers ink to a porous printing form, preferably having a ceramic exterior surface, which transfers the ink to a rubber blanket cylinder which prints the image from the printing form cylinder onto a web substrate.
What is needed is an improved, simplified method and roll system to meter coatings on moving metallic substrates such as aluminum alloy sheet at various coating speeds, which would provide long runs before applicator roll change and uniform, flawless coatings on the substrate. What is unique about coating metal webs is the constantly varying width, gauge, and coating requirements. If all production parameters were similar, the coating application system could be optimized for it. For example, a constant width web might make slot coating the preferred method. As it is, frequent product changes are a fact of life now. If the same width were run all the time, a soft backup (impression) roll could be used. If the same coating were run all the time optimization of a direct forward gravure coater, like in the aluminum foil industry, or slot die coater for magnetic tape could be used. The reason roll coating has such a strong presence in metal finishing is its flexibility and low equipment cost. If the applicator roll is more rigorous, productivity increases. Gravure rolls accommodate about a 20% film weight variation with a single coating. Roll coating, however, is less subject to surface tension and substrate surface energy than gravure coating. Slot coating has potential for metallic substrates except the possibility for die damage is high and width adjustment is questionable. Attainment of  less than 2.0 micrometer film weights while maintaining coating properties has proven to be difficult with extrusion coating.
It is one of the main objects of this invention to provide a simplified method to coat metal objects or substrates of sheet thickness ( greater than 100 micrometers) with or without using deformable rolls, where the method reduces production downtime and provides defect-free coatings.
These and other objects are accomplished by providing a method of coating a metal sheet substrate having a thickness greater than 0.1 mm comprising: providing a continuously moving sheet metal substrate having a thickness greater than 0.1 mm; providing a source of coating liquid; applying the coating liquid to a micro-porous, flat ceramic surfaced applicator roll, and contacting the metal sheet substrate directly with the coated, micro-porous, flat ceramic surfaced applicator roll to coat the substrate.
This method utilizes a hard-surfaced, non-deformable applicator roll against a thick, but roll bendable, hard, non-deformable metal substrate. This method can be a single or multiple pass process using one or more metering rolls or other pre-metering equipment to uniformly coat the ceramic surfaced applicator roll. The substrate can be support by a non-deformable backup roll opposite the applicator roll where the substrate passes between them, or the substrate can be supported by two or more passline rolls where the applicator roll is disposed between the passline rolls. The substrate is a non-deformable flat metal sheet having a thickness of from 0.1 mm to 0.8 mm and the coating rate/speed is from about 40 m/min to 500 m/min. The coating liquid can be a solution of protective resin, lubricating material, surface enhancing material, cosmetic enhancing material, and the like. These coatings can be heated at the end of the process, if necessary, at a temperature effective to cure or otherwise activate them, herein defined as xe2x80x98curexe2x80x99. The metering roll surface can be deformable, such as polyurethane. The applicator roll can have a wide range of average surface roughness (Ra) from about 0.03 to 35 micrometers, depending on the film weight that must be transferred to the substrate and the line speed. At higher line speeds, a high Ra value applicator roll may introduce air into the metering nip of a multi-roll coating configuration, therefore pre-metered delivery systems such as slot die, curtain, or enclosed coating feed chambers may be employed to prevent air entrainment in the bulk coating or applied coating. Within the above Ra range the ceramic roll will provide uniform, defect-free coating to the metallic web. This applicator roll directly contacts the moving substrate, allowing reduction of the number of rolls needed, while the applicator roll applies a pressure on the substrate of from about 0.3 kg/cm2 to 3.0 kg/cm2 (4 psi to 40 psi). The wrap angle of the metallic web contacting the applicator roll is between about 0 and 15 degrees, preferably between 2 and 10 degrees, where the wrap angle is the difference between where the sheet exits the applicator roll from an extended line from the incoming side of the web.
The invention also resides in a coated metal sheet substrate having a thickness greater than 0.1 mm, made by providing a continuously moving metal substrate having a thickness greater than 0.1 mm; providing a source of coating liquid; metering the coating liquid on to a micro-porous, flat ceramic surfaced applicator roll; and contacting the metal substrate directly with the coated, applicator roll, to coat the substrate. The metal is preferably aluminum alloy sheet. The invention is not applicable to xe2x80x9cmetal foilxe2x80x9d which is generally defined as having a thickness less than 100 micrometer (0.001 mm).
This invention may utilize a deformable metering roll, but now the metering roll is removed from direct contact with the metallic web and is not subject to the same wear. As such, the applicator roll, contacting the metal sheet web, can have a higher durometer rating, which either lowers metering nip forces (longer roll life) or allows one to apply higher solids coatings (cost savings) at the same dry film thickness. The usual wet coating thickness is from about 5 to 100 micrometers.